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Collagen chemistry and the brittle bone diseases
Collagen chemistry and the brittle bone diseases Colin R. Paterson Brittle bone diseases have been recognised for more than 100 years, but the last ten years have seen remarkable advances in our understanding of their causes. There are many defects of collagen that can cause this type of problem. Most of them have a genetic basis, and in some the fundamental defect in the DNA coding for collagen has now been identified. A few brittle bone diseases, notably scurvy and copper deficiency, have a nutritional basis
Pamela was not a happy baby. From the time of her birth she would scream whenever she was handled, particularly when a nappy was being changed. On several occasions her parents were reassured that all was well, but eventually she was referred to hospital, where it was found that both of her femurs were broken. There was evidence of healing and it seemed likely that the fractures had occurred at birth. Additional fractures, of one ankle, both upper arms, one forearm, and one rib came to light at the same time. Most of these had probably also occurred at, or not long after, birth but there was no known history of trauma. Since that time she has had further fractures, in situations in which other children would not have fractured. None of the early fractures were recognised until the radiographs had been done. At no time did Pamela have bruises, weals, or contusions that might have been expected had these fractures been sustained as a result of the extensive and repeated trauma that would be needed to cause fractures in normal bones. Osteogenesis
impetfecta
Pamela has osteogenesis imperfecta (‘brittle bone disease’) and is likely to continue to have fractures throughout childhood. This is not a new disease; a 4000 year old skull from Upper Nubia in the British Museum shows good evidence that the child had this disorder. One of the leaders of the Danish invasion of England in the Dark Ages was Colin R Paterson F.R.C. Path
M.A.,
M.Sc.,
D.M.,
M.R.C.P.,
Qualified in medicinefrom Oxfordand University College Hospital in 1962. He has been interested in bone disease since 1964 and is the author of Metabolic Disorders of Bone, (1975). He is now senior lecturer in biochemical medicine at Dundee University; his current research fields include risk factors for osteoporosis and the diagnosis of bone diseases in children.
Endeavour, New Series, Volume 12, No. 2, 1988. 018~9327188 $3.00 + .OO. Pergamon Press pk. Printed in Great Britain.
56
Ivar the Boneless, who was said to have been carried into battle on a shield. Osteogenesis imperfecta affects all races; it is reported among the Chinese in Hong Kong, in Indians in Britain, and in blacks in the USA. In Western Europe and North America it affects about one person in 20 000. Not all are equally affected. Some are severely affected and sustain multiple fractures at, or even before, birth. At the other extreme are children who have only two or three fractures in the whole of childhood, and who would not have been correctly diagnosed but for the presence of other features of the disease or a positive family history. It is now clear that osteogenesis imperfecta is not a single disease but a group of at least ten disorders, all almost certainly resulting from inborn abnormalities of the fibrous protein collagen. In addition, a temporary brittle bone disease results from defective collagen formation in scurvy (vitamin C deficiency) and in copper deficiency. Collagen is the principal protein of bone, but its importance for the strenth of bone is not always recognised. It forms about 30 per cent of the weight of bone, most of the remainder being the calcium-containing crystal hydroxyapatite. Collagen provides a threedimensional lattice-work within bone, giving it strength in tension as does steel in reinforced concrete bridges. Unlike the hydroxyapatite, it is not radioopaque and, as the amount of hydroxyapatite is normal in many patients with osteogenesis imperfecta, the radiographic image of the bone may appear normal despite its fragility (figure 1). While fractures are the most obvious expression of collagen disorders, defects in other collagen-containing tissues also occur. It is an essential constituent of tendons, ligaments, joint capsules, teeth, skin, blood vesselsand the sclerae (the whites of the eyes). Many patients therefore have other clinical abnormalities, including increased joint flexibility (figure 2) and therefore increased liability to sprains and dislocations, abnormal
teeth (figure 3), and a liability to bruising (figure 4). A blue-grey discolouration of the sclerae may occur; this reflects the thinness of the sclerae with the pigment ‘showing through’ (figure 5). Few patients have all these signs and some, undoubtedly affected on the basis of family history, have none. Such is the extent to which the disorder varies from person to person. The current classification of the disorder identifies three main groups of patients. First, there are infants with a very severe disease who are either stillborn or die in the first few days of life. Secondly, there are infants with a severe disease, usually with fractures at birth, with progressive deformity, and with greatly impaired growth (figure 6). The third group comprises patients with a mild or moderate disease and a very variable number of fractures. Each group has been subdivided; in the caseof the ‘mild’ disease the subdivision depends on the presence or absenceof blue sclerae and abnormal teeth. This classification is of more than academic interest, since competent genetic counselling depends on correct classification. The mild disease is generally inherited in a ‘dominant’ manner; that is, a child is affected after acquiring a single abnormal gene from one parent who is also affected (figure 7). However the same disorder can arise de nova as a result of a ‘new mutation’. The severe disease is most usually inherited in a recessive manner; that is, both parents have one abnormal gene in a particular pair but are not overtly affected and the child has the disorder as a result of inheriting both abnormal genes (figure 8). The genetics of the very severe disorder are not fully understood; some patients have a recessively inherited disease, some probably result from ‘new mutations’, while in others the genetic pattern is not yet clear. The classification of the disorder is important for a second reason. The clinical differences reflect differences in the fundamental structure of collagen. Recent years have seen great advances
in our understanding of collagen formation, and of the many different steps at which errors can take place. It may be helpful therefore to summarise collagen chemistry and synthesis. Collagen Collagen is the principal structural protein of bone, skin, tendons and liga-
Figure 4 Extensive spontaneous bruising with normal activity in a child with osteogenesis imperfecta.
Figure 1 (a) Normal bone radiographic appearances at the time of the first fracture of a tibia in a 3-year-old boy with a mild inherited form of osteogenesis imperfecta. (b) The same bone three years (and four fractures) later. The appearances are now clearly abnormal as a result of the fractures and of the immobilisation used in their treatment.
Figure 2 Increased joint flexibility child with osteogenesis imperfecta.
in a
ments. Collagen fibres show a characteristic appearance on electron microscopy (figure 9), and detailed study shows that this appearance reflects the arrangement of collagen molecules, each consisting of three polypeptide chains (figure 10). At least twelve different collagens are recognised in different tissues, but the collagen of bone is type I collagen, and consists of two identical CL~chains and one G[?chain. Collagen molecules are unusual proteins chemically in that they contain a large proportion of the very small amino acid, glycine, and also two unusual amino acids, hydroxyproline and hydroxylysine. The glycine component appears to be essential for allowing the tight helical arrangement of the three chains. Hydroxyproline and hydroxylysine appear to be important for the links between chains and between molecules. Collagen formation is a comprlex process including both intracellular and
Figure 6 Radiograph of legs of a child with severe osteogenesis imperfecta. Many previous fractures have led to deformity of all the bones. Unlike the patient shown in figure 1 this child’s radiographs were abnormal from birth.
extracellular steps. As with other proteins, the synthesis takes place on the rough endoplasmic reticulum, under the control of messenger RNA transcribed from DNA in the nucleus. The translation of the growing peptide chain leads it into the lumen of the rough endoplasmic reticulum where the initial portion, the signal segment, is removed. The subsequent steps are illustrated in figure 11 and include the enzymatic modification of certain proline and lysine residues to give hydroxyproline and hydroxylysine residues, the addition of hexose units, and the aggregation of two aI and one CI?chains to give the triple helix. These are then held together by ‘di-
TI I
‘T”
l--f--l
I
‘T”
r-l
q-5J--hI, .- h Figure 3 Abnormal teeth in a child with one form of osteogenesis imperfecta.
Figure 5 Blue-grey sclerae in an adult with osteogenesis imperfecta.
Figure 7 Pedigree of a family with mild dominately inherited osteogenesis imperfecta. n 0 = males, 0 0 = females, n 0 = affected, q 0 = unaffected.
57
T
distorted chain leads to delay in later maturation. It now appears that the severity of the brittle bone disease is greatest when the cysteine substitution is nearest the carboxy-terminal of the protein. This is thought to relate to the fact that the formation of the helix starts at the carboxy-terminal end. The nearer the abnormality is to this end, the greater the length of helix that is abnormal. Recent work has shown that several separate genetic defects may cause a disease with similar clinical manifestations. However, the situation is more complex still in that apparently identical biochemical defects can cause different clinical problems. For example, three cases of patients with absent a2 chains Figure 8 Pedigree of a family to show recessive inheritance of a severe form of have been described. One had a modosteogenesis imperfecta. In this family there were two consanguineous marriages erately severe brittle bone disease, and (marriages between close relatives, indicated by =) and the presumed carriers of the two had the Ehlers-Danlos syndrome. abnormal gene are shown as 0 and 0. Each of these individuals was clinically Even within a family the individuals with normal. an apparently identical biochemical defect may show wide variations in the degree to which they are overtly sulphide bonds’ between cysteine re- these enzymes, or as an acquired disease affected. It has been suggestedthat these sidues. At this stage the procollagen resulting from lack of the co-factors variations result from variations in other molecule is extruded from the cell and needed for their action. In recent years, features of the genetic make up. further modification takes place, with most of these types of defect have been the removal of the non-helical peptides identified. Some have effects so severe Acquired defects in collagen at each end of the molecule and the that the affected infant is stillborn with synthesis. multiple fractures; others lead to severe At least two nutritional disorders are aggregation of the molecules into fibrils. brittle bone diseasewith survival; others associated with brittle bones. These are lead to mild brittle bone disease; and yet vitamin C deficiency (scurvy) and copper Collagen abnormalities in brittle further abnormalities cause disorders of deficiency. Both vitamin C and copper bone disease. There are many stages at which it is collagen-containing tissues but without are needed for steps in the maturation of possible for errors to affect this process. bone disease. The best known disorder collagen: vitamin C, it is thought, for the Mutations may affect the genes coding in the last group is the Ehlers-Danlos formation of disulphide bridges between for either al or c(z chains (on chromo- syndrome, a group of diseasescharacte- peptide chains, and copper as a composomes 17 and 7 respectively). There rised by increased flexibility of joints and could be defects in the processing of the abnormal elasticity of the skin. * The best recognised chemical abnor303nm messenger RNA and there could be I* _-_defects in any of the enzymes responsi- mality. in osteogenesis imperfecta is a f ble for the post-translational modifica- diminution of the production of type I 1.5nm tion of the collagen precursors. These collagen by cultured cells. It is becoming 1 mm _--‘\ // could arise either as an inherited disease clear that this diminution has many \ from defects of the genes coding for distinct causes, including reduced production of the c(i procollagen chain, failure of production of the a2 procollagen chain, and defective release of procollagen from cells. One way in which we can be confident that most cases of the milder (familial) forms of osteogenesis imperfecta result from collagen defects, is through linkage studies. If large families are studied, it can be demosntrated that the osteogenesis imperfecta is inherited along with markers for the genes coding for either the al or the G(~procollagen chains. The recent remarkable advances in molecular biology have led to the identification of specific mutations in DNA coding for procollagen. One type of Figure 10 Structure of a collagen fibril. mutation appears to be responsible for The collagen molecule consists of a helix. The molecules aggregate several cases. This consists of the re- triple outside the cell into fibrils in a placement of one of the very small ‘quarter-stagger’ arrangement in such a glycine residues with a much larger cys- way that the sequence of hole zones and teine residue. This abnormality causes overlap zones accounts for the banded Figure 9 Electron microscopic distortion of the triple helix which is appearance of collagen on electron appearance of collagen fibres from skin. greater if the cysteine side chams are in microscopy (lowest figure, courtesy of The characteristic cross-banding is seen the centre of the helix (figure 12). The Prof. J. W. Smith). (Courtesy of Dr D. A. Hall).
\ \\
\\\
/
/ / /
N-terminal
C-terminal .
Figure 13 Radiograph of left leg of a child with scury to show the healing fracture of the upper part of the tibia and widespread bleeding under the periosteum (the covering of the bone shaft).
D Figure 11 Steps in the formation of a collagen molecule. A: synthesis of the peptide chains from amino acids by the ribosomes on the rough endoplasmic reticulum. Hydroxylation of certain proline and lysine residues to hydroxyproline and hydroxylysine takes place at this stage, as does the addition of certain hexose units (a, 0, A, n l. 6 and C: formation of the triple helix of procollagen starting at the carboxy-terminal (C-terminal) end. After the procollagen has been extruded from the cell the two terminal peptides are removed to give a collagen molecule. (After D. Prockop, New England Journal of Medicine 301, 13, 1979).
Figure 12 Deformation of the triple helix (seen end-on) as a result of the replacement of the small glycine residues by the large cysteine residues 0. It is suggested that a severe (lethal) disease results from the replacement of two adjacent glycine residues as a result of a mutation affecting the ~1, chains. A milder disease might result from a similar mutation affecting the single cfz chain. (Courtesy of Dr 6. Steinmann).
(‘premature’) birth seems to be an important risk factor for copper deficiency; this probably relates to the fact that copper stores in the fetus are largely built up in the last third of pregnancy. Another risk factor appears to be twin pregnancy, perhaps because the available copper supply is then divided benent of the enzyme lysyl oxidase essentween two. A third risk factor appears to tial for the formation of crosslinks bebe bottle-feeding; the disorder is very tween molecules. rare in breast-fed infants. Scurvy is mainly characterised by In copper deficiency the formation of bruising, bleeding from the gums, and defective blood-vessel collagen may bleeding under the covering of the bones cause bleeding under the periosteum or, (the periosteum), all thought to be due less commonly, bruising. Fractures and to defective collagen (and therefore fraother bone changes occur; characteristigility) of small blood vessels. Fractures cally, as in other brittle bone disorders, do occur both in children and in adults the fractures occur with little or no with scurvy (figure 13). The whole disorder improves rapidly when vitamin c1 known trauma, and most are found ‘by accident’ when radiographs are done for is administered. other reasons. Some of the other recogCopper deficiency has been reported nised abnormalities of copper deficiency in some 60 infants over the last 30 years, can also be attributed to defects in almost entirely within the first year of copper-containing enzymes. Anaemia, life. It is probably a great deal more for example, is common; it results from common than these small numbers defects in haemoglobin formation and would imply, and occurs particularly in from increased destruction of red cells. infants born before full term. Preterm
Some affected infants have episodes of apnoea (failure to breathe) and some die suddenly perhaps as a result. The cause of this abnormality is not clear. Acknowledgement The author acknowledges the help of Dr S. J. McAllion and Mr J. Burns in the preparation of this review and the support of the Rehabilitation and Medical Research Trust in his work on osteogenesis imperfecta. Bibliography [II Byers, P. H. and Bondaio. J. F. ‘The molecular basis of clinical heterogeneity in osteogenesis imperfecta: mutations in type I collagen genes have different effects on collagen processing.’ In ‘Genetic and Metabolic Disease in Pediatrics.’ Lloyd, J. K. and Striver, C. R. (eds.). Butterworths, London. 1985. PI Prockop, D. J. ‘Mutations in collagen genes: consequences for rare and common diseases.’ J. Clin. Invest. 75, 783787, 1Y85. 131 Sykes, B.. Ogilvie, D., Wordsworth, P., Anderson, J. and Jones, N. ‘Osteogenesis imperfecta is linked to both type I collagen structural genes.’ LUKE; i. 69-72. i986. [41 Sykes, B. C. and Smith, K. ‘Collagen and collagen gene disorders’. Quurt. J. Med. 56,533s547,19x5.
[5] Wallis, G., Beighton, P., Boyd, C. and Mathew. C. G. ‘Mutations linked to the pro c(, (I) collagen gene are responsible for several cases of osteogenesis imperfecta type I’, J. Med. Genet. 23, 411-16; 1986.
59
Pamela was not a happy baby. From the time of her birth she would scream whenever she was handled, particularly when a nappy was being changed. On several occasions her parents were reassured that all was well, but eventually she was referred to hospital, where it was found that both of her femurs were broken. There was evidence of healing and it seemed likely that the fractures had occurred at birth. Additional fractures, of one ankle, both upper arms, one forearm, and one rib came to light at the same time. Most of these had probably also occurred at, or not long after, birth but there was no known history of trauma. Since that time she has had further fractures, in situations in which other children would not have fractured. None of the early fractures were recognised until the radiographs had been done. At no time did Pamela have bruises, weals, or contusions that might have been expected had these fractures been sustained as a result of the extensive and repeated trauma that would be needed to cause fractures in normal bones. Osteogenesis
impetfecta
Pamela has osteogenesis imperfecta (‘brittle bone disease’) and is likely to continue to have fractures throughout childhood. This is not a new disease; a 4000 year old skull from Upper Nubia in the British Museum shows good evidence that the child had this disorder. One of the leaders of the Danish invasion of England in the Dark Ages was Colin R Paterson F.R.C. Path
M.A.,
M.Sc.,
D.M.,
M.R.C.P.,
Qualified in medicinefrom Oxfordand University College Hospital in 1962. He has been interested in bone disease since 1964 and is the author of Metabolic Disorders of Bone, (1975). He is now senior lecturer in biochemical medicine at Dundee University; his current research fields include risk factors for osteoporosis and the diagnosis of bone diseases in children.
Endeavour, New Series, Volume 12, No. 2, 1988. 018~9327188 $3.00 + .OO. Pergamon Press pk. Printed in Great Britain.
56
Ivar the Boneless, who was said to have been carried into battle on a shield. Osteogenesis imperfecta affects all races; it is reported among the Chinese in Hong Kong, in Indians in Britain, and in blacks in the USA. In Western Europe and North America it affects about one person in 20 000. Not all are equally affected. Some are severely affected and sustain multiple fractures at, or even before, birth. At the other extreme are children who have only two or three fractures in the whole of childhood, and who would not have been correctly diagnosed but for the presence of other features of the disease or a positive family history. It is now clear that osteogenesis imperfecta is not a single disease but a group of at least ten disorders, all almost certainly resulting from inborn abnormalities of the fibrous protein collagen. In addition, a temporary brittle bone disease results from defective collagen formation in scurvy (vitamin C deficiency) and in copper deficiency. Collagen is the principal protein of bone, but its importance for the strenth of bone is not always recognised. It forms about 30 per cent of the weight of bone, most of the remainder being the calcium-containing crystal hydroxyapatite. Collagen provides a threedimensional lattice-work within bone, giving it strength in tension as does steel in reinforced concrete bridges. Unlike the hydroxyapatite, it is not radioopaque and, as the amount of hydroxyapatite is normal in many patients with osteogenesis imperfecta, the radiographic image of the bone may appear normal despite its fragility (figure 1). While fractures are the most obvious expression of collagen disorders, defects in other collagen-containing tissues also occur. It is an essential constituent of tendons, ligaments, joint capsules, teeth, skin, blood vesselsand the sclerae (the whites of the eyes). Many patients therefore have other clinical abnormalities, including increased joint flexibility (figure 2) and therefore increased liability to sprains and dislocations, abnormal
teeth (figure 3), and a liability to bruising (figure 4). A blue-grey discolouration of the sclerae may occur; this reflects the thinness of the sclerae with the pigment ‘showing through’ (figure 5). Few patients have all these signs and some, undoubtedly affected on the basis of family history, have none. Such is the extent to which the disorder varies from person to person. The current classification of the disorder identifies three main groups of patients. First, there are infants with a very severe disease who are either stillborn or die in the first few days of life. Secondly, there are infants with a severe disease, usually with fractures at birth, with progressive deformity, and with greatly impaired growth (figure 6). The third group comprises patients with a mild or moderate disease and a very variable number of fractures. Each group has been subdivided; in the caseof the ‘mild’ disease the subdivision depends on the presence or absenceof blue sclerae and abnormal teeth. This classification is of more than academic interest, since competent genetic counselling depends on correct classification. The mild disease is generally inherited in a ‘dominant’ manner; that is, a child is affected after acquiring a single abnormal gene from one parent who is also affected (figure 7). However the same disorder can arise de nova as a result of a ‘new mutation’. The severe disease is most usually inherited in a recessive manner; that is, both parents have one abnormal gene in a particular pair but are not overtly affected and the child has the disorder as a result of inheriting both abnormal genes (figure 8). The genetics of the very severe disorder are not fully understood; some patients have a recessively inherited disease, some probably result from ‘new mutations’, while in others the genetic pattern is not yet clear. The classification of the disorder is important for a second reason. The clinical differences reflect differences in the fundamental structure of collagen. Recent years have seen great advances
in our understanding of collagen formation, and of the many different steps at which errors can take place. It may be helpful therefore to summarise collagen chemistry and synthesis. Collagen Collagen is the principal structural protein of bone, skin, tendons and liga-
Figure 4 Extensive spontaneous bruising with normal activity in a child with osteogenesis imperfecta.
Figure 1 (a) Normal bone radiographic appearances at the time of the first fracture of a tibia in a 3-year-old boy with a mild inherited form of osteogenesis imperfecta. (b) The same bone three years (and four fractures) later. The appearances are now clearly abnormal as a result of the fractures and of the immobilisation used in their treatment.
Figure 2 Increased joint flexibility child with osteogenesis imperfecta.
in a
ments. Collagen fibres show a characteristic appearance on electron microscopy (figure 9), and detailed study shows that this appearance reflects the arrangement of collagen molecules, each consisting of three polypeptide chains (figure 10). At least twelve different collagens are recognised in different tissues, but the collagen of bone is type I collagen, and consists of two identical CL~chains and one G[?chain. Collagen molecules are unusual proteins chemically in that they contain a large proportion of the very small amino acid, glycine, and also two unusual amino acids, hydroxyproline and hydroxylysine. The glycine component appears to be essential for allowing the tight helical arrangement of the three chains. Hydroxyproline and hydroxylysine appear to be important for the links between chains and between molecules. Collagen formation is a comprlex process including both intracellular and
Figure 6 Radiograph of legs of a child with severe osteogenesis imperfecta. Many previous fractures have led to deformity of all the bones. Unlike the patient shown in figure 1 this child’s radiographs were abnormal from birth.
extracellular steps. As with other proteins, the synthesis takes place on the rough endoplasmic reticulum, under the control of messenger RNA transcribed from DNA in the nucleus. The translation of the growing peptide chain leads it into the lumen of the rough endoplasmic reticulum where the initial portion, the signal segment, is removed. The subsequent steps are illustrated in figure 11 and include the enzymatic modification of certain proline and lysine residues to give hydroxyproline and hydroxylysine residues, the addition of hexose units, and the aggregation of two aI and one CI?chains to give the triple helix. These are then held together by ‘di-
TI I
‘T”
l--f--l
I
‘T”
r-l
q-5J--hI, .- h Figure 3 Abnormal teeth in a child with one form of osteogenesis imperfecta.
Figure 5 Blue-grey sclerae in an adult with osteogenesis imperfecta.
Figure 7 Pedigree of a family with mild dominately inherited osteogenesis imperfecta. n 0 = males, 0 0 = females, n 0 = affected, q 0 = unaffected.
57
T
distorted chain leads to delay in later maturation. It now appears that the severity of the brittle bone disease is greatest when the cysteine substitution is nearest the carboxy-terminal of the protein. This is thought to relate to the fact that the formation of the helix starts at the carboxy-terminal end. The nearer the abnormality is to this end, the greater the length of helix that is abnormal. Recent work has shown that several separate genetic defects may cause a disease with similar clinical manifestations. However, the situation is more complex still in that apparently identical biochemical defects can cause different clinical problems. For example, three cases of patients with absent a2 chains Figure 8 Pedigree of a family to show recessive inheritance of a severe form of have been described. One had a modosteogenesis imperfecta. In this family there were two consanguineous marriages erately severe brittle bone disease, and (marriages between close relatives, indicated by =) and the presumed carriers of the two had the Ehlers-Danlos syndrome. abnormal gene are shown as 0 and 0. Each of these individuals was clinically Even within a family the individuals with normal. an apparently identical biochemical defect may show wide variations in the degree to which they are overtly sulphide bonds’ between cysteine re- these enzymes, or as an acquired disease affected. It has been suggestedthat these sidues. At this stage the procollagen resulting from lack of the co-factors variations result from variations in other molecule is extruded from the cell and needed for their action. In recent years, features of the genetic make up. further modification takes place, with most of these types of defect have been the removal of the non-helical peptides identified. Some have effects so severe Acquired defects in collagen at each end of the molecule and the that the affected infant is stillborn with synthesis. multiple fractures; others lead to severe At least two nutritional disorders are aggregation of the molecules into fibrils. brittle bone diseasewith survival; others associated with brittle bones. These are lead to mild brittle bone disease; and yet vitamin C deficiency (scurvy) and copper Collagen abnormalities in brittle further abnormalities cause disorders of deficiency. Both vitamin C and copper bone disease. There are many stages at which it is collagen-containing tissues but without are needed for steps in the maturation of possible for errors to affect this process. bone disease. The best known disorder collagen: vitamin C, it is thought, for the Mutations may affect the genes coding in the last group is the Ehlers-Danlos formation of disulphide bridges between for either al or c(z chains (on chromo- syndrome, a group of diseasescharacte- peptide chains, and copper as a composomes 17 and 7 respectively). There rised by increased flexibility of joints and could be defects in the processing of the abnormal elasticity of the skin. * The best recognised chemical abnor303nm messenger RNA and there could be I* _-_defects in any of the enzymes responsi- mality. in osteogenesis imperfecta is a f ble for the post-translational modifica- diminution of the production of type I 1.5nm tion of the collagen precursors. These collagen by cultured cells. It is becoming 1 mm _--‘\ // could arise either as an inherited disease clear that this diminution has many \ from defects of the genes coding for distinct causes, including reduced production of the c(i procollagen chain, failure of production of the a2 procollagen chain, and defective release of procollagen from cells. One way in which we can be confident that most cases of the milder (familial) forms of osteogenesis imperfecta result from collagen defects, is through linkage studies. If large families are studied, it can be demosntrated that the osteogenesis imperfecta is inherited along with markers for the genes coding for either the al or the G(~procollagen chains. The recent remarkable advances in molecular biology have led to the identification of specific mutations in DNA coding for procollagen. One type of Figure 10 Structure of a collagen fibril. mutation appears to be responsible for The collagen molecule consists of a helix. The molecules aggregate several cases. This consists of the re- triple outside the cell into fibrils in a placement of one of the very small ‘quarter-stagger’ arrangement in such a glycine residues with a much larger cys- way that the sequence of hole zones and teine residue. This abnormality causes overlap zones accounts for the banded Figure 9 Electron microscopic distortion of the triple helix which is appearance of collagen on electron appearance of collagen fibres from skin. greater if the cysteine side chams are in microscopy (lowest figure, courtesy of The characteristic cross-banding is seen the centre of the helix (figure 12). The Prof. J. W. Smith). (Courtesy of Dr D. A. Hall).
\ \\
\\\
/
/ / /
N-terminal
C-terminal .
Figure 13 Radiograph of left leg of a child with scury to show the healing fracture of the upper part of the tibia and widespread bleeding under the periosteum (the covering of the bone shaft).
D Figure 11 Steps in the formation of a collagen molecule. A: synthesis of the peptide chains from amino acids by the ribosomes on the rough endoplasmic reticulum. Hydroxylation of certain proline and lysine residues to hydroxyproline and hydroxylysine takes place at this stage, as does the addition of certain hexose units (a, 0, A, n l. 6 and C: formation of the triple helix of procollagen starting at the carboxy-terminal (C-terminal) end. After the procollagen has been extruded from the cell the two terminal peptides are removed to give a collagen molecule. (After D. Prockop, New England Journal of Medicine 301, 13, 1979).
Figure 12 Deformation of the triple helix (seen end-on) as a result of the replacement of the small glycine residues by the large cysteine residues 0. It is suggested that a severe (lethal) disease results from the replacement of two adjacent glycine residues as a result of a mutation affecting the ~1, chains. A milder disease might result from a similar mutation affecting the single cfz chain. (Courtesy of Dr 6. Steinmann).
(‘premature’) birth seems to be an important risk factor for copper deficiency; this probably relates to the fact that copper stores in the fetus are largely built up in the last third of pregnancy. Another risk factor appears to be twin pregnancy, perhaps because the available copper supply is then divided benent of the enzyme lysyl oxidase essentween two. A third risk factor appears to tial for the formation of crosslinks bebe bottle-feeding; the disorder is very tween molecules. rare in breast-fed infants. Scurvy is mainly characterised by In copper deficiency the formation of bruising, bleeding from the gums, and defective blood-vessel collagen may bleeding under the covering of the bones cause bleeding under the periosteum or, (the periosteum), all thought to be due less commonly, bruising. Fractures and to defective collagen (and therefore fraother bone changes occur; characteristigility) of small blood vessels. Fractures cally, as in other brittle bone disorders, do occur both in children and in adults the fractures occur with little or no with scurvy (figure 13). The whole disorder improves rapidly when vitamin c1 known trauma, and most are found ‘by accident’ when radiographs are done for is administered. other reasons. Some of the other recogCopper deficiency has been reported nised abnormalities of copper deficiency in some 60 infants over the last 30 years, can also be attributed to defects in almost entirely within the first year of copper-containing enzymes. Anaemia, life. It is probably a great deal more for example, is common; it results from common than these small numbers defects in haemoglobin formation and would imply, and occurs particularly in from increased destruction of red cells. infants born before full term. Preterm
Some affected infants have episodes of apnoea (failure to breathe) and some die suddenly perhaps as a result. The cause of this abnormality is not clear. Acknowledgement The author acknowledges the help of Dr S. J. McAllion and Mr J. Burns in the preparation of this review and the support of the Rehabilitation and Medical Research Trust in his work on osteogenesis imperfecta. Bibliography [II Byers, P. H. and Bondaio. J. F. ‘The molecular basis of clinical heterogeneity in osteogenesis imperfecta: mutations in type I collagen genes have different effects on collagen processing.’ In ‘Genetic and Metabolic Disease in Pediatrics.’ Lloyd, J. K. and Striver, C. R. (eds.). Butterworths, London. 1985. PI Prockop, D. J. ‘Mutations in collagen genes: consequences for rare and common diseases.’ J. Clin. Invest. 75, 783787, 1Y85. 131 Sykes, B.. Ogilvie, D., Wordsworth, P., Anderson, J. and Jones, N. ‘Osteogenesis imperfecta is linked to both type I collagen structural genes.’ LUKE; i. 69-72. i986. [41 Sykes, B. C. and Smith, K. ‘Collagen and collagen gene disorders’. Quurt. J. Med. 56,533s547,19x5.
[5] Wallis, G., Beighton, P., Boyd, C. and Mathew. C. G. ‘Mutations linked to the pro c(, (I) collagen gene are responsible for several cases of osteogenesis imperfecta type I’, J. Med. Genet. 23, 411-16; 1986.
59